The present invention is directed to a noise reduction system, more particularly, a multiple parallel input noise reduction system for eliminating noise in order to detect small input signals.
In many different fields, it is frequently necessary to detect and measure very small input signals and for preventing the transmission of noise. In fact, in many instances, the noise signals themselves may even be larger than the input signals which it is desired to detect and measure. For example, electrical signals are generated as a result of the conduction system of the heart of the human body. These electrical signals are very low level signals which are ordinarily not detectable by the usual recording means because of the magnitude of the interfering noise. Similarly, it is often desirable to detect other small biologic signals like those arising from the central nervous system as well as signals in the field of space exploration, sound engineering, electron spin resonance devices, photochemical measurements, communication systems, etc. The sensitivity of any recording system is limited by the level of input noise as well as the amplifier noise which is generated in any high fidelity recording of small input signals.
A number of different techniques are known in the art for reducing noise in low level signal measurements. For example, computer techniques have been developed to improve signal to noise ratio by applying serial signal/digital averaging techniques. Signal averaging techniques can be used to measure signals which are smaller than the noise signals generated in amplifiers. These latter techniques generally use the classical cross correlation principle which is generally known in the art. A general discussion of such a noise reduction technique can be found in the article "How To Measure Noise That Is Quieter Than Your Pre-Amp", Application Note 127 (1974), Princeton Applied Research Corporation, Princeton, New Jersey.
The signal averaging techniques implemented by computer systems have a number of major disadvantages. First of all, these systems lack the ability to detect moment-to-moment changes in the true input signal configuration, thereby introducing errors in amplitude, duration, etc. of the input signal. Also, these systems have a low efficiency rating. For example, if a signal has 100 Hz components and the period from the reference signal varies with the standard deviation of plus or minus 0.45 milliseconds, the resultant amplitude of the digitally averaged signal will be approximately half the true amplitude of the original signal with no noise contamination. Finally, signal averaging systems cannot generate on-line information because of the inevitable delay implicit in the signal sampling and averaging technique. Moreover, the necessary storage elements required in implementing the signal averaging technique complicates the design of such systems and greatly adds to the cost of their construction.
In U.S. Pat. No. 3,126,449 issued to J. Shirman on Mar. 24, 1964, a noise discriminator circuit is shown having a logic circuit for opening a gate to pass the input signal in the event that the input has certain frequency characteristics. The logic circuitry includes a plurality of parallel channels for passing signals having the selected frequency characteristics and logically combining these signals to open the gate to pass the input signal. As a result, this circuit discriminates against input signal noise.
Several other patents also show related noise reduction systems. For example, U.S. Pat. No. 2,920,281 issued to Appert et al discloses a circuit for reducing noise by connecting a pair of amplifiers in parallel. In this regard, it is well known in the art that the parallel connection of a plurality of amplifiers, transistors, etc. reduces input noise by a factor corresponding to the number of parallel amplifiers, transistors, etc. Peak level detectors for detecting the positive and/or negative peak levels of impulsive noise in the input signal are also known in the art as illustrated by U.S. Pat. No. 4,156,202 issued to Takahashi on May 22, 1979.